The long-term objective of this research is to gain a better understanding of the regulation of skin development. The goal of this project is to elucidate the function of Foxn1, a member of the winged- helix/forkhead family of transcription factors. In rodents, the loss of Foxn1 function results in the nude phenotype, which is characterized by the abnormal morphogenesis of the skin, thymus, mammary gland, and nails. Based on our work to date, we have developed the following model of Foxn1 function. In the skin, epithelial cells induce Foxn1 as they lose the ability to multiply and initiate terminal differentiation. In a site- dependent manner, Foxn1 then promotes up to three developmental processes: the differentiation of its host cells, the melanization of its host cells by melanocytes, and the growth of its surrounding host epithelium. To regulate these processes, Foxn1 activates intercellular signaling systems, thereby inducing epithelial cells to cooperate with each other and pigment cells. According to this model therefore, Foxn1 acts as a regulatory nexus, coordinating the growth, differentiation, and pigmentation of cutaneous tissues. To explain how this Foxn1 nexus works, we have developed the following hypothesis: that Foxn1 organizes cells into cooperative units by activating two types of extracellular signals - diffusible signals, which are responsible for Foxn1's long-range actions (e.g., the induction of cell proliferation and the recruitment of melanocytes to epithelial cells), and cell-bound signals, which are responsible for Foxn1's short-range actions (e.g., the flagging of epithelial cells for pigmentation and the locking of pigmentary connections into place). To test this hypothesis and the underlying model, the downstream effectors of Foxn1 will be elucidated. The specific aims of the project are as follows: 1) to isolate and characterize Foxn1 effectors, 2) to determine the significance of the effectors to the Foxn1 pathway and skin, and 3) to dissect the molecular mechanisms by which Foxn1 regulates its effectors. In humans, FOXN1 is conserved in sequence and function, suggesting it employs a conserved set of downstream effectors. Thus, by identifying Foxn1 effectors, the project should provide insight into disorders associated with the abnormal growth, differentiation, or activity of melanocytes and epithelial cells.